The present invention is directed to a method of analyzing organic additives in electroplating baths. More specifically, the present invention is directed to a method of analyzing organic additives in electroplating baths having improved reproducibility.
Electroplating baths for copper and other metals are typically aqueous, or mostly aqueous, solutions composed of metal compounds or salts, ionic electrolytes, and various additives such as brighteners, suppressors, levelers, accelerators, surfactants, and defoamers. These electroplating baths, which are used to deposit metals or semimetals such as copper, nickel, gold, palladium, platinum, ruthenium, rhodium, tin, zinc, antimony, or alloys such as copper-tin (brass), copper-zinc (bronze), tin-lead, nickel-tungsten, and cobalt-tungsten-phosphide are used in applications such as the fabrication of electronic devices and components, such as conductive circuits for printed circuit boards, multichip modules, and semiconductor devices.
Reliable operation of these electroplating baths in a manufacturing process requires that methods of analysis are employed to determine the appropriate concentrations of the reagent species for bath startup. These analytical methods also are used to determine the concentrations of species in the bath during operation, often with on-line feedback control, to allow the components of the bath to be monitored and adjusted as required to maintain concentrations within pre-determined limits. Bath analytical methods also are used to determine the chemical identity and concentrations of species that are produced in the bath as a consequence of chemical and electrochemical reactions that take place during bath operation and/or idling.
Electrochemical methods are used principally for the analysis of acid copper plating baths used for plating circuitry on printed wiring boards and integrated circuits. Besides the inorganic components of these plating solutions (copper ions, sulfuric acid and small amounts of chloride ions) the baths contain one or more organic additives (brighteners, suppressors and levelers). In the proper concentrations, these organic additives give a bright, smooth deposit with excellent mechanical and electrical properties.
Analyses of plating bath additives are described by Tench and coworkers in U.S. Pat. No. 4,132,605, and Fisher in U.S. Pat. Nos. 4,917,774 and 4,917,777. These methods were devised to measure the brightener concentration in a bath containing a suppressor and inorganic components only. These methods cannot measure the presence of a leveler component in a plating bath containing brightener and suppressor.
The electrochemical methods for plating bath analysis described by Tench et al. and Fisher rely on the fact that the brightener and suppressor work in opposition to one another with respect to their effect on the potential of an object being plated. Suppressors, as their name implies, increase the overpotential for plating and thus suppress the plating rate for any given electrical energy input to the bath. In the presence of suppressors, brighteners lower the plating overpotential and cause the plating rate to increase for any given input of electrical energy to the plating bath. Suppressors cause an abrupt suppression of the plating rate at very low concentrations, on the order of 50 parts per million or less. Above that threshold level the plating overpotential changes very little, if at all. Suppressor concentrations are usually kept in the range of several hundreds to several thousand parts per million to ensure that the suppressor concentration is always well above the threshold value.
U.S. Pat. Nos. 4,917,774 and 4,917,777 essentially describe a stepped potential method where a three electrode cell is employed. The electrodes are 1) a working electrode, 2) a counter electrode, and 3) a reference electrode, which are all immersed in the plating solution to be analyzed. The working electrode is a noble metal such as platinum in the form of a rotating disc. The disc is sealed at one end of a Kel F rod and is rotated during the analysis to ensure that uniform hydrodynamic conditions prevail. The potential of the working electrode is controlled by input from a potentiostat slaved to a computer. For any working potential required, the computer will direct the potentiostat to set the potential difference between the counter electrode and the reference electrode to give the desired potential at the working electrode. The computer and potentiostat can be embodied in a single unit such as the Electroposit™ Bath Analyzer, (Shipley Company, Marlborough, Mass. and S-Sytems, Norwood, Mass.). The potential of a working electrode is held at various potentials in a plating solution to clean, equilibrate, plate and strip the plated deposit from the electrode. A measurement of the initial current flowing during the plating step is directly related to the brightener concentration in the plating solution. When using the Electroposit™ Bath Analyzer, the initial plating current is displayed as Total Brightener Analysis Units (“TBA”) units, hereinafter referred to as TBA analysis.
Levelers often are present in plating baths. Like suppressors, levelers cause a reduction of plating rate for any given electrical energy input to the plating bath. Unlike suppressors, whose effect is general in nature, levelers cause a localized depression in plating rate. They act under mass transfer control to suppress the plating rate by adsorbing at locally higher potential regions of the article being plated. The above described electrochemical methods cannot be used when levelers are present in the bath. This is due to the leveler's suppressing effect on the plating rate, which varies with its concentration in the bath. To properly analyze for brightener when leveler is present the leveler concentration must already be known. The techniques described above provide no solution for overcoming the confounding effect of the leveler on the brightener, and therefore cannot be used when these two additives coexist in a plating bath.
U.S. Pat. No. 5,223,118 to Sonnenberg et al. describes an analysis method that measures both brightener and leveler coexisting with suppressor in a plating solution. In this method the brightener concentration is first determined by the TBA method and, if necessary, adjusted by external addition to a value that gives maximum sensitivity for analysis of leveler. The method then uses a freshly prepared copper coated electrode to monitor the energy input with time to the electrode while plating at an applied current held at a constant value. Such copper coated electrode is prepared by first plating copper from a separate copper electroplating solution free of organic additive on an electrode of a different metal. The electrode typically is platinum. The slope of the resulting energy-time plot is used to quantitatively determine the leveler concentration.
To repeat the analysis method all of the copper is removed from the platinum by a nitric etch, which also removes adsorbed bath components from the electrode. The copper also may be removed with an anodic potential/current. Nevertheless, a nitric acid etch is still needed to remove any adsorbed species on the electrode. The nitric acid etch and the electrochemical stripping of copper from the electrode cleans the electrode of varying amounts of metal oxide and metal chloride on the electrode surface. Such oxides on the electrode surface alter the plating potential and steps for forming a copper film on the electrode, which in turn affects the adsorption behavior of organic additives. Such surface inconsistencies compromise the reproducibility of measuring organic bath additives and increase the error in determining concentrations of the organic additives.
While the Fisher and Sonnenberg patents are improvements over the Tench methods for analyzing organic additives for plating baths, workers in the art are always seeking improved analytical methods that provide measurements that are more reproducible, thus reducing the amount of error. Reducing error in the measurement of organic additives of electroplating baths is highly desirable in the electroplating industry. An undesirable decline or increase in organic additives during electroplating processes may result in a defective end product, thus increasing the cost and decreasing the efficiency of the manufacture of articles prepared by electroplating. Many organic additives in electroplating baths, such as brighteners and levelers, are employed in relatively low concentrations. Accordingly, changes in bath concentrations of such organic additives have a pronounced detrimental affect on the quality of the final product.
Accordingly, there is a continuing need for the analysis of brightener and leveler concentrations in plating baths that works over a wide range of brightener concentrations, is simple to perform and does not require the addition of other components to the bath for the procedure to be performed and provides reproducible measurements.